When Charles Darwin visited the remote Galápagos archipelago in 1835, the observations he made of the islands' various species of long-lived tortoises would play a pivotal role in helping inform his theory of evolution.

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One of those species, the massive Chelonoidis elephantopus was long believed to have gone extinct just a few years after Darwin's visit. But now, scientists have used a clever genetic trick to show that several C. elephantopus are likely still alive.

How do you find a species long believed to be extinct? Simple: look for traces of its presence. For many people, that would mean looking for tracks, droppings, the distribution of the animal's resources, or the animal itself. To Yale biologists Ryan Garrick and Edgar Benavides, however, that meant scouring Isabela Island — the largest in the Galápagos archipelago — for traces of genetic evidence.

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Garrick, Benavides, and their team took blood samples from close to 1700 tortoises (about 20% of the island's population) inhabiting the northern tip of the island. The tortoises belong to a species known today as Chelonoidis becki. When the researchers compared the DNA from their samples with DNA extracted from the bones of museum specimens of C. elephantopus, they found that many of the modern day C. becki tortoises were, in fact, hybrids — they actually contained traces of "extinct" C. elephantopus DNA (pictured up top is one such example of a C. becki hybrid.)

In fact, 84 of the samples contained so much C. elephantopus DNA, that the team concluded at least one recent common ancestor must have been a pure-bred member of the presumably extinct species (in other words, the researchers were dealing with first-generation hybrids); and in thirty cases, say the researchers, breeding likely took place within the last 15 years.

Given that these tortoises have been known to live well beyond 100 years of age, Garrick and Benavides believe there is a high probability that purebred C. elephantopus are still alive and kicking somewhere on the island as you read this.

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"To our knowledge," write Garrick and Benavides in the latest issue of Current Biology, this is the first rediscovery of a species by way of tracking the genetic footprints left in the genomes of its hybrid offspring."